Many of recent electronic devices improve an integration degree in a circuit by using a printed wiring board in which a plurality of insulating layers and a plurality of conductive layers (wiring layers) are stacked. In this case, a wiring formed in each wiring layer is usually connected to a hole penetrating all layers in the board, by a conductor (referred to as a “through-hole” in the following) with which the inner surface of the hole is plated.
In the field of data communication such as Ethernet (Registered Trademark), a data forwarding speed becomes higher year by year. Recently, particularly, to increase a speed of transmitting an electric signal, differential signals, e.g., have been used. This is an idea for reducing influence of a noise from an outside, and maintaining completeness of a high-frequency electric signal that propagates through a transmission path.
As a higher frequency is advanced, an electric signal is more affected by discontinuity of impedance in a transmission line (wiring). As a result, the tendency that a loss of a signal, a noise, and a cross talk between wirings occur becomes high. Accordingly, to maintain electric performance obtained by a signal using a high frequency, it is desired to make impedance substantially constant over an entire transmission line.
However, a through-hole formed in the above-described printed wiring board penetrates the printed wiring board entirely. For this reason, a parasitic stub will be formed at a connection point with a signal wiring formed in a wiring layer. It is known that this parasitic stub produces discontinuity of impedance in the transmission line, and becomes a factor of a signal loss and the like. 10 Gbase-KR defined in IEEE802.3ap is Ethernet (Registered Trademark) disposed via a backplane. It is known that in the case of 10 Gbase-KR, a loss caused by a stub parasitic on a through-hole for mounting a backplane connector is particularly large.
FIG. 9 is a first diagram illustrating a configuration of a printed wiring board related to an exemplary embodiment of the present invention.
In the following, the above-described parasitic stub is briefly explained. As illustrated in FIG. 9, a printed wiring board 2 is configured by stacking many ground layers and wiring layers that are placed on each other via insulating layers. The ground layers and the wiring layers are each a conductor layer. FIG. 9 is depicted with omission of the insulating layers for the convenience of explanation, but actually, the insulating layer made of a dielectric substance such as a resin exists between the respective conductor layers.
At the ground layer, a ground plane 202 formed as a conductor pattern is provided.
The ground plane 202 functions as a ground. At the wiring layer, a signal wiring 203 formed also as a conductor pattern is provided. On the basis of the conductor pattern, the signal wiring 203 functions as a transmission line that propagates a predetermined electric signal.
Provided in the printed wiring board 2 are grounding through-holes 200a to 200c that penetrate the board 2 and connect to all the ground planes 202 provided at a plurality of the ground layers. The grounding through-holes 200a to 200c are written generically as grounding through-holes 200.
Likewise, provided in the printed wiring board 2 are signal through-holes 201a to 201d that penetrate the board 2 and connect to at least one of the signal wirings 203 provided at a plurality of the wiring layers. The signal through-holes 201a to 201d are written generically as signal through-holes 201. The signal through-hole 201 functions as a transmission line that plays a role of making connection to the signal wiring 203 and propagating an electric signal.
The signal through-holes 201a and 201b are connected to differential-signal signal wirings that transmit electric signals whose phases are inverted from each other. As illustrated in FIG. 9, the grounding through-holes 200a and 200b are arranged beside the signal through-holes 201a and 201b so as to form pairs with the signal through-holes 201a and 201b, respectively.
A clearance 204 is formed by hollowing out the conductor planes 202 of the ground layers. The clearance 204 is a non-conductor pattern for making a state where the ground planes 202 and the signal through-hole 201 are physically separated from each other. As illustrated in FIG. 9, the clearance 204 is formed around the signal through-holes 201a and 201b that propagate differential signals. Meanwhile, to make a configuration in which the ground planes 202 and the grounding through-hole 200 are physically contacted with each other, the clearance 204 is not formed around the grounding through-hole 200 at the all layers.
FIG. 10 is a second diagram illustrating a configuration of the printed wiring board 2 related to an exemplary embodiment of the present invention.
FIG. 10 is a diagram showing a sectional schematic view of the printed wiring board 2 illustrated in FIG. 9. As illustrated in FIG. 10, into each of the through-holes 201 and 202, a corresponding signal pin of an integrated circuit (IC) is inserted. The insulating layer 205 exists between the respective conductor layers, and electrically insulates the conductor layers from each other.
As illustrated in FIG. 10, the signal through-holes 201a and 201b penetrate the printed wiring board 2 from the upper surface to the lower surface. The signal through-holes 201a and 201b are connected to the signal wirings 203A of the first wiring layer from the upper side in the 20 printed wiring board 2, and are not connected to the second and following wiring layers at all. Accordingly, the signal through-holes 201a and 201b form “stub St” as illustrated in FIG. 10.
The stub St becomes a cause to increase “parasitic capacitance Pc” between the stub St and the conductor planes 202 as illustrated in FIG. 10. In a transmission line of an electric signal, parasitic capacitance Pc increases by existence of the stub St. As a result, impedance in the transmission line locally decreases. This leads to a cause for impedance mismatch that is not intended by a designer, and causes degradation in quality of high-frequency signal propagating through the transmission line.
As a method for dealing with the problem that the stub St is formed in the signal through-hole 201, there is a method called “back drilling method”. The back drilling method is a method that cuts off and removes a parasitic stub portion in the through-hole. By this back drilling method, a parasitic stub can be physically removed.
As a measure for the above-described problem, patent literature 1 discloses a method for connecting ground planes to a grounding through-hole that makes potentials uniform by connecting the ground planes of respective ground layers with each other. The patent literature 1 discloses, as one of connecting methods, a method that connects the grounding through-hole only to the ground planes of the ground layers neighboring the wiring layer.